Oryzacystatin we found in the seed of rice (J. Biol. Chem. 262, 16793-16797 (1987)) has been recognized as the first well-defined cystatin (proteinaceous cysteine proteinase inhibitor) of plant origin (Biomed. Biochim. Acta 50, 637-641 (1991). While oryzacystatin has its own endogenous target enzyme "oryzain" in the rice seed (J. Biol. Chem. 266, 16897-16902 (1991)), it can also target some exogenous cysteine proteinases to be carried by invading insects. Actually, we have confirmed that the addition of oryzacystatin to experimental diets for Sitophilus oryzae (Kokuzomushi) and Riptortus clavatus (Kamemushi) is effective in regulation of their growth. The effective amounts of oryzacystatin added in these cases are small enough for this inhibitor to receive practical application to insect regulation. We are now going to extend this line of study in order to verify the regulatory effect on other kinds of cereal insects.A new attempt is being made together. It involves molecular cloning of a cysteine proteinase of an insect which occurs in its digestive tract (midgut). The cloning is successful and it will be soon possible to obtain this enzyme by recombinant DNA techniques. We then plan to analyze the inhibition of this exogenous enzyme by oryzacystatin from a kinetic point of view for detailed understanding of the inhibitory mechanism on the molecular level.These basic as well as applied studies will provide both academic and practical information about this plant cystatin as a new, safe insect-regulator.